Vacuum circuit interrupters are, generally, constructed of a highly evacuated envelope, a stationary electrical contact provided in the envelope, a movable electrical contact provided in the envelope so as to be opposite to the stationary electrical contact and shields. The envelope comprises, substantially, a tubular housing and a pair of end plates. The housing is, generally, fabricated by a cylindrical insulating material, and a pair of metallic end plates used to form the evacuated envelope.
Moreover, the vacuum interrupter is, generally, constructed by the steps of fixing an upper and a lower end plate to each axial end of a cylindrical insulating housing respectively, mounting a bellows on the lower end plate, inserting a movable contact rod into the bellows, securing a movable electrical contact on the movable contact rod, and incorporating a stationary contact rod (secured a stationary electrical contact at the bottom thereof) to the upper end plate.
In this conventional vacuum interrupter it is difficult to make a diameter of the envelope large because the envelope is very expensive when a large diameter housing made of glass or ceramics is used as a part of the envelope. Further, a material made of an alloy Fe-Ni-Co or an alloy of Fe-Ni is usually employed in a vacuum-tight seal. This vacuum-tight seal is also very expensive and has a magnetic property. The vacuum interrupter, therefore, becomes very expensive and is low in reliability because the temperature thereof rises due to the eddy current generated by high current flow of the contact rods. Further, mechanical strength of the envelope is lower when a diameter of the housing is larger, and therefore the vacuum interrupter becomes low in reliability.